The Green twins? (4)
I believe the answer is:
peas
'the green twins?' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'green twins?' is the wordplay.
'green' becomes 'pea' (pea green is a kind of green).
'twins?' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'pea'+'s'='PEAS'
Can you help me to learn more?
(Other definitions for peas that I've seen before include "Pulses" , "Seeded vegetables" , "Podded vegetables" , "pulse" , "Edible seeds" .)